Where there's not much traffic from your end then you get to keep the same IP?
I am using 56U direct on MIF also but my IP stays the same, are there any system logs from your 56U when the problem happens?
Apr 17 20:22:40 linkstatus: WAN link down detected!
Apr 17 20:22:44 linkstatus: WAN link restored!
Apr 17 20:22:46 linkstatus: Perform WAN DHCP renew...
Apr 17 20:22:50 linkstatus: WAN link down detected!
Apr 17 20:22:52 linkstatus: WAN link restored!
Apr 17 20:22:54 linkstatus: Perform WAN DHCP renew...
Apr 17 20:22:54 RT-N56U: wan down (eth3)
Apr 17 20:22:54 dhcp client: deconfig: lease is lost
Apr 17 20:23:15 wan: perform manual connect
Apr 17 20:23:15 stop_wan(): perform DHCP release
Apr 17 20:23:16 kernel: eth3: ===> VirtualIF_close
Apr 17 20:23:16 kernel: GDMA2_MAC_ADRH -- : 0x000084c9
Apr 17 20:23:16 kernel: GDMA2_MAC_ADRL -- : 0xb2bb86f7
Apr 17 20:23:16 dnsmasq[328]: read /etc/hosts - 4 addresses
Apr 17 20:23:16 dnsmasq[328]: using nameserver 8.8.4.4#53
Apr 17 20:23:16 dnsmasq[328]: using nameserver 8.8.8.8#53
Apr 17 20:23:16 kernel: GDMA2_MAC_ADRH -- : 0x000084c9
Apr 17 20:23:16 kernel: GDMA2_MAC_ADRL -- : 0xb2bb86f7
Apr 17 20:23:16 kernel: eth3: ===> VirtualIF_open
Apr 17 20:23:16 dhcp client: starting wan dhcp (eth3) ...
Apr 17 20:23:16 RT-N56U: wan down (eth3)
Apr 17 20:23:16 dhcp client: deconfig: lease is lost
Apr 17 20:23:16 RT-N56U: wan up (eth3)
Apr 17 20:23:16 RT-N56U: Hardware NAT: Enabled, IPv4/PPPoE offload [WAN]<->[LAN]
Apr 17 20:23:16 RT-N56U: Software FastNAT: Disabled
Apr 17 20:23:16 dhcp client: bound (eth3), IP: 182.55.112.211, GW: 182.55.112.1, lease time: 14400
Apr 18 00:09:17 stop_wan(): perform DHCP release
Apr 18 00:09:17 RT-N56U: wan down (eth3)
Apr 18 00:09:17 dnsmasq[3522]: read /etc/hosts - 4 addresses
Apr 18 00:09:17 dnsmasq[3522]: using nameserver 218.186.2.6#53
Apr 18 00:09:17 dnsmasq[3522]: using nameserver 218.186.2.16#53
Apr 18 00:09:17 dnsmasq[3522]: using nameserver 218.186.1.58#53
Apr 18 00:09:17 dhcp client: deconfig: lease is lost
Apr 18 00:09:17 kernel: eth3: ===> VirtualIF_close
Apr 18 00:09:18 kernel: rtl8367m - create vlan: pvid=[1071], prio=[1], member=[0x031F], untag=[0x030F], fid=[0]
Apr 18 00:09:18 kernel: GDMA2_MAC_ADRH -- : 0x000084c9
Apr 18 00:09:18 kernel: GDMA2_MAC_ADRL -- : 0xb2bb86f7
Apr 18 00:09:18 kernel: GDMA2_MAC_ADRH -- : 0x000084c9
Apr 18 00:09:18 kernel: GDMA2_MAC_ADRL -- : 0xb2bb86f7
Apr 18 00:09:18 kernel: eth3: ===> VirtualIF_open
Apr 18 00:09:18 dhcp client: starting wan dhcp (eth3) ...
Apr 18 00:09:18 RT-N56U: wan down (eth3)
Apr 18 00:09:18 dnsmasq[3522]: read /etc/hosts - 4 addresses
Apr 18 00:09:18 dnsmasq[3522]: using nameserver 8.8.8.8#53
Apr 18 00:09:18 dhcp client: deconfig: lease is lost
Apr 18 00:09:18 RT-N56U: wan up (eth3)
Apr 18 00:09:18 dnsmasq[3522]: read /etc/hosts - 4 addresses
Apr 18 00:09:18 dnsmasq[3522]: using nameserver 218.186.2.6#53
Apr 18 00:09:18 dnsmasq[3522]: using nameserver 218.186.2.16#53
Apr 18 00:09:18 dnsmasq[3522]: using nameserver 218.186.1.58#53
Apr 18 00:09:18 RT-N56U: Hardware NAT: Enabled, IPv4/PPPoE offload [WAN]<->[LAN]
Apr 18 00:09:18 RT-N56U: Software FastNAT: Disabled
Apr 18 00:09:19 dhcp client: bound (eth3), IP: 182.55.112.80, GW: 182.55.112.1, lease time: 14400
Apr 18 00:09:44 dnsmasq[3522]: exiting on receipt of SIGTERM
Apr 18 00:09:44 dnsmasq[3827]: started, version 2.60 cachesize 1000
Apr 18 00:09:44 dnsmasq[3827]: compile time options: no-IPv6 GNU-getopt no-RTC no-DBus no-i18n no-IDN DHCP no-DHCPv6 no-scripts no-TFTP no-conntrack
Apr 18 00:09:44 dnsmasq-dhcp[3827]: DHCP, IP range 192.168.100.2 -- 192.168.100.200, lease time 1d
Apr 18 00:09:44 dnsmasq[3827]: read /etc/hosts - 4 addresses
Apr 18 00:09:44 dnsmasq[3827]: using nameserver 218.186.2.6#53
Apr 18 00:09:44 dnsmasq[3827]: using nameserver 218.186.2.16#53
Apr 18 00:09:44 dnsmasq[3827]: using nameserver 218.186.1.58#53
4 hours